An investigation of the mortality rates of young adults born in the postwar period of large-scale atmospheric nuclear testing (1945-1965) in the United States and other western industrial nations reveals an increasingly anomalous rise in mortality from its previous secular decline. Beginning in the late 1970s and particularly since 1983, the deterioration in the health of the 25-44 age group is related to in utero exposure to fission products in the milk and diet, associated with an unprecedented rise in underweight births and neonatal mortality known to be accompanied by loss of immune resistance. The 1945-1965 rise in the percentage of live births below 2500 grams is highly correlated with the amount of strontium-90 in human bone, both peaking in the mid-1960s. In the 1980s, for the baby boom generation (those born between 1945 and 1965), cancer incidence and mortality due to infectious diseases associated with a rising degree of immune deficiency, such as pneumonia, septicemia, and AIDS, increased sharply. This process of increasing immune deficiency appears to have been exacerbated by continuing secondary exposures to accidental reactor releases and by an acceleration of radiation-induced mutation of pathogenic microorganisms increasingly resistant to drugs.